(236a) Flow-Induced Density Gradients In Microscale Flows

We report the results of non-equilibrium
molecular dynamics simulations of pressure-driven flows of liquid argon in
planar conduits.  At equilibrium our
molecular dynamics results are in excellent agreement with those found in the
literature.  In the simulations, the
conduits were ~ 60 atomic diameters across with ~25,000 atoms.  Simple shear flows were generated by moving
the walls of the channel and the pressure driven flows were generated by
applying a body force to each of the atoms ranging from 0.01 to 0.1 pN.  When the flow rates are very small, the
liquid is on the average incompressible and a parabolic profile as predicted by
incompressible creeping flow equations is observed.  However, as the flow rates increases in
isothermal simulations, we find that in the pressure-driven flows the molecules
migrate to the low-shear-rate region in the center of the conduits and
establish large radial density gradients under conditions that were previously
assumed to be incompressible. The magnitude of the density gradients in the
inhomogeneous shear flows increase monotonically with flow rate in the conduit
under conditions such that there is no significant slip at the walls.  The migration of the atoms to the center of
the flow channels results in a blunted velocity profile that deviates from the
solutions to the compressible Navier-Stokes equations that predict a parabolic
velocity profile and a density that is only a function of the axial position in
the channel. When these same systems are subjected to linear simple shear flow
at the same shear rates, aside from localized wall-induced order, there is no
variation of the average density across the channel.  Hence this phenomenon is associated with the
inhomogeneous or nonlinear shear in the pressure driven flows and not the
magnitude of the shear rate. These results are distinctly different than
simulations in which the walls were isothermal and viscous heating leads to
density decrease in the center of the channel.